docs/markdown/Rust.md

---
title: Rust
short-description: Working with Rust in Meson
...

# Using Rust with Meson

## Avoid using `extern crate`

Meson can't track dependency information for crates linked by rustc as
a result of `extern crate` statements in Rust source code.  If your
crate dependencies are properly expressed in Meson, there should be no
need for `extern crate` statements in your Rust code, as long as you use the
Rust 2018 edition or later. This means adding `rust_std=2018` (or later) to the
`project(default_options)` argument.

An example of the problems with `extern crate` is that if you delete a
crate from a Meson build file, other crates that depend on that crate
using `extern crate` might continue linking with the leftover rlib of
the deleted crate rather than failing to build, until the build
directory is cleaned.

This limitation could be resolved in future with rustc improvements,
for example if the [`-Z
binary-dep-depinfo`](https://github.com/rust-lang/rust/issues/63012)
feature is stabilized.

## Mixing Rust and non-Rust sources

*(Since 1.9.0)* Rust supports mixed targets, but only supports using
`rustc` as the linker for such targets. If you need to use a non-Rust
linker, or support Meson < 1.9.0, see below.

Until Meson 1.9.0, Meson did not support creating a single target with
Rust and non Rust sources mixed together. One had to compile a separate
Rust `static_library` or `shared_library`, and link it into the C build
target (e.g., a library or an executable).

```meson
rust_lib = static_library(
    'rust_lib',
    sources : 'lib.rs',
    rust_abi: 'c',
    ...
)

c_lib = static_library(
    'c_lib',
    sources : 'lib.c',
    link_with : rust_lib,
)
```

## Mixing Generated and Static sources

*Note* This feature was added in 0.62

You can use a [[@structured_src]] for this. Structured sources are a dictionary
mapping a string of the directory, to a source or list of sources.
When using a structured source all inputs *must* be listed, as Meson may copy
the sources from the source tree to the build tree.

Structured inputs are generally not needed when not using generated sources.

As an implementation detail, Meson will attempt to determine if it needs to copy
files at configure time and will skip copying if it can. Copying is done at
build time (when necessary), to avoid reconfiguring when sources change.

```meson
executable(
    'rust_exe',
    [[#structured_sources]](
        'main.rs',
        {
            'foo' : ['bar.rs', 'foo/lib.rs', generated_rs],
            'foo/bar' : [...],
            'other' : [...],
        }
    )
)
```

## Use with rust-analyzer

*Since 0.64.0.*

Meson will generate a `rust-project.json` file in the root of the build
directory if there are any rust targets in the project. Most IDEs will need to
be configured to use the file as it's not in the source root (Meson does not
write files into the source directory). [See the upstream
docs](https://rust-analyzer.github.io/book/non_cargo_based_projects.html) for
more information on how to configure that.

### Clippy
You can use the "clippy-json" build target as rust-analyer's "check command" to recieve clippy diagnostics in your editor. 

Without overriding the check command, the LSP will function in a limited state, only showing certain errors (for example, no borrow checking errors are shown).

[Non cargo based projects](https://rust-analyzer.github.io/book/non_cargo_based_projects.html) shows how to override the check command, you probably want to set it to `ninja clippy-json -C build`.

## Linking with standard libraries

Meson will link the Rust standard libraries (e.g. libstd) statically, unless the
target is a proc macro or dylib, or it depends on a dylib, in which case [`-C
prefer-dynamic`](https://doc.rust-lang.org/rustc/codegen-options/index.html#prefer-dynamic)
will be passed to the Rust compiler, and the standard libraries will be
dynamically linked.

## Building static libraries for `no_std` environments

Meson by default links all the dependencies of `libstd` into executables that depend
on Rust `staticlib`s.  Specifying `b_freestanding=true` lets Meson know that the
`staticlib` uses `#[no_std]`, and reduces the set of libraries that are linked into
its dependencies.

## Multiple targets for the same crate name

For library targets that have `rust_abi: 'rust'`, the crate name is derived from the
target name.  First, dashes, spaces and dots are replaced with underscores.  Second,
*since 1.10.0* anything after the first `+` is dropped.  This allows creating multiple
targets for the same crate name, for example when the same crate is built multiple
times with different features, or for both the build and the host machine.

## Compiler vs. linker arguments for Rust

While `rustc` integrates the compiler and linker phase, it is useful
to pass linker arguments to it via the `-Clink-arg=` command line
option.

*Since 1.11.0* `add_project_link_arguments()`,
`add_global_link_arguments()`, the `link_args` keyword argument wrap the
arguments with `-Clink-arg=` before passing them to the Rust compiler.
Furthermore, these arguments are only included when creating binary or
shared library crates.  Likewise, methods such as `has_link_argument()`
wrap the arguments being tested with `-Clink-arg=`.

## Interaction between `rust_panic=abort` and testing

Cargo and Meson build tests differently.  `cargo test` recompiles the whole
dependency tree from scratch using the dedicated `test` profile, so the test
build is wholly independent from the artifacts produced by `cargo build`.
Meson instead reuses the libraries that were already built and only compiles an
additional test-harness binary out of the crate's own sources.

Each approach has tradeoffs.  Rebuilding everything lets the `test` profile
differ arbitrarily from the `dev` profile, but it is slow and, in the common
case where the two profiles are essentially the same, it duplicates a lot of
work for no benefit.  Reusing the libraries is much faster and detects bugs
caused by e.g. compiler optimizations; but it requires the test harness and
the libraries it links against to be ABI compatible.

This has a consequence for the `rust_panic` option, and therefore for the
`panic` key of a Cargo `[profile]`.  Rust's test harness relies on catching
panics in order to report failures, so it must be compiled with the default
`unwind` strategy; `rust.test()` and `rust.doctest()` honor this by resetting
`rust_panic` to `none` for the harness even when the rest of the build uses
`rust_panic=abort`.  The harness recompiles the crate's own sources with
`unwind`, but `rustc` refuses to link it against a dependency that was compiled
with `-Cpanic=abort`:

```
error: the linked panic runtime `panic_unwind` is not compiled with this crate's panic strategy `abort`
```

As a result, a crate built with `rust_panic=abort` is effectively
untestable unless it has no dependencies.  Therefore, `rust_panic=abort`
is only recommended if you are building a simple `cdylib` or `staticlib`
with most or all of your dependencies in subprojects.  In that case,
you can test the dependencies by building the subprojects on their own.

## Cargo interaction

*Since 1.11.0*

In most cases, a Rust program will use Cargo to download crates.  Meson is able
to build Rust library crates based on a `Cargo.toml` file; each external crate
corresponds to a subproject.  Rust modules that do not need a `build.rs` file
need no intervention, whereas if a `build.rs` file is present it needs to be
converted manually to Meson code.

To enable automatic configuration of Cargo dependencies, your project must
have `Cargo.toml` and `Cargo.lock` files in the root source directory;
this enables proper feature resolution across crates.  You can then
create a workspace object using the Rust module, and retrieve specific
packages from the workspace:

```meson
rust = import('rust')
cargo_ws = rustmod.workspace()
anyhow_dep = ws.subproject('anyhow').dependency()
```

The workspace object also enables configuration of Cargo features, for example
from Meson options:

```meson
cargo_ws = rustmod.workspace(
    features: ['feature1', 'feature2'])
```

Finally, the workspace object is able to build targets specified in `lib`
or `bin` sections, extracting compiler arguments for dependencies and
diagnostics from the Cargo.toml file.  The simplest case is that of building
a simple binary crate:

```meson
cargo_ws.package().executable(install: true)
```

For a workspace:

```meson
pkg_lib = cargo_ws.package('myproject-lib')
lib = pkg_lib.library(install: false)
pkg_lib.override_dependency(declare_dependency(link_with: lib))

cargo_ws.package().executable(install: true)
```

Sources are automatically discovered, but can be specified as a
[[@structured_src]] if they are partly generated.

It is still possible to use keyword arguments to link non-Rust build targets,
or even to use the usual Meson functions such as [[static_library]] or
[[executable]].

### Non-default and wildcard workspace members

With Cargo, workspace members that are not included in
`workspace.default-members` can be accessed through the `-p` option.
In Meson, non-default members are not part of dependency resolution
by default; the set of optional packages that has to be configured
is specified *since 1.12.0* in the `extra_members` argument of the
`workspace()` method, while up to Meson 1.11.x non-default members would
never be built.

Cargo subprojects will only build default members unless they have a custom
(handwritten) `meson.build` that sets `extra_members` appropriately;
typically the choice of which members to build will come from a Meson
option.

Furthermore, while Cargo accepts glob patterns for the `workspace.members`
field, Meson (*since 1.12.0*) expands such patterns but never treats
globbed patterns as default members.  Therefore, they will only be built
if included in `extra_members`.

Note that crates that come from `crates.io` are never workspaces, and
therefore they are not subject to these differences.

### Subprojects with separate workspaces

*Since 1.13.0* Meson honors the `workspace.exclude` field.  Directories
listed there are not workspace members, and therefore they are not
returned by the `packages()` method nor accepted by `package()`.  As with
Cargo, excluding a directory also excludes everything below it, entries
are not treated as glob patterns, and a member that is listed literally
in `workspace.members` takes precedence over `workspace.exclude`.

This is useful, for example, when path dependencies come from subprojects
that have their own workspace and `Cargo.lock` files.  Because Meson
only supports one `Cargo.lock` file per project, excluded members must
be subprojects.